John Tsang asks...."If I am defending at a ruck, can I move off the defensive line as soon as the scrum half have his hands on the ball?"
Or to put it another way........When is the ball out of the ruck?
The IRB have helpfully cleared this up for us once and for all. The ball is out when it is lifted off the floor (or beyond the back foot of the ruck). So to answer John's questions, NO! You cannot move in front of the offside line when the scrum half has his hands on the ball. You must wait until he lifts the ball clear of the floor.
Offside at the ruck. When a scrum half attempts to retrieve the ball from a ruck, the ball is not out until that player has picked the ball up from the ground. In the clip the scrum half is taken out by the player before the ball is off the ground and this would be deemed to be offside by the player tackling the scrum half. If, however, the scrum half had picked up the ball and a defending player tackles the scrum half, that player does so without sanction.
See the video here: IRB Law Application Guidelines
The Rugby Ref